Switching amplifiers, also known as Class-D amplifiers, have the advantages in efficiency and benefits because of the efficiency, when compared to the Class-AB amplifiers. As shown in FIG. 1, a conventional switching amplifier 10 with a low-pass filter (LPF) comprises an H-bridge circuit composed of transistors M1, M2, M3 and M4, and a control circuit 12 in response to an input signal Vi to switch the transistors M1, M2, M3 and M4 to thereby generate a pair of output signals OUTP and OUTN. The operation of the conventional switching amplifier 10 makes the output signals OUTP and OUTN in opposite polarities at any input signal and any time, and therefore, when the switching amplifier 10 is connected directly with an inductive load 16 such as a coil type speaker, there will be a large switching current flowing through the load 16, which will induce severe EMI problems and have the load 16 to bear the power dissipation resulted from the induced signals outside of the audio frequency band. It is thus necessary to filter out the signal components outside of the audio frequency band from the output signals OUTP and OUTN by a LPF 14 before they are applied to the load 16. FIG. 2. shows a waveform diagram of the output signals OUTP and OUTN of the switching amplifier 10 at different input signal Vi's, in which waveform 20 represents the output signal OUTP and waveform 22 represents the output signal OUTN. When the input signal Vi=0, the duty cycles of the output signals OUTP and OUTN both are 50%; when the input signal Vi>0, the duty cycle of the output signal OUTP increases and the duty cycle of the output signal OUTN decreases; and when the input signal Vi<0, the duty cycle of the output signal OUTP decreases and the duty cycle of the output signal OUTN increases. In other conventional switching amplifiers, it may also be the case that when the input signal Vi>0, the duty cycle of the output signal OUTP decreases while the duty cycle of the output signal OUTN increases, and when the input signal Vi<0, the duty cycle of the output signal OUTP increases while the duty cycle of the output signal OUTN decreases.
However, introducing the LPF 14 will result in cost raise and efficiency drop for implementing a switching amplifier, and it is therefore provided another type of amplifier, i.e., filterless switching amplifier. Under appropriate switching control, for instance disclosed in U.S. Pat. Nos. 6,211,728 and 6,262,632, a filterless switching amplifier may retain the advantages in efficiency. Additionally, the EMI issue is also important in the design of filterless switching amplifiers. The present invention is directed to a high efficiency and low EMI switching amplifier and a control method thereof.